Some conventionally known motor vehicles designed to be straddled by riders, such as under-bone type two-wheeled and three-wheeled motorcycles, have electronically-controlled fuel injection devices in place of carburetors in the intake systems of engines. For example, the under-bone type motorcycles described in Patent Documents 1 and 2 have engines attached under the backbones. In the motorcycles, a fuel injection device is positioned downstream of a throttle valve, in an intake passage that communicates to an intake valve opening of the engine.
FIG. 14 is apartial side view showing an example of a conventional under-bone type motorcycle equipped with an engine having a fuel injection device (see Patent Document 1). FIG. 14 shows the intake system partially in cross-section.
The under-bone type motorcycle 1 shown in FIG. 14 has a backbone 3 that extends rearward and obliquely downward from a head pipe 2. The head pipe 2 turnably supports a steering shaft, with handles attached on top of the steering shaft.
An engine 4 is positioned under the rear part of the backbone 3. An air cleaner 5 is positioned under the front part of the backbone 3. The air cleaner 5 is connected to an intake port 4a of the engine 4 through an intake pipe 6. A throttle body 7 is interposed in the intake pipe 6.
The part of the intake pipe 6 that is connected to the intake port 4a is bent, and a fuel injection device 8 is attached to the bent part in such a direction that the fuel injection device 8 injects fuel toward an intake valve opening 4b. 
In order to prevent thermal damage from the engine 4, the fuel injection device 8 is positioned in the rear part of the intake pipe 6 above the engine 4 as distant as possible from the intake valve (the intake valve opening 4b) of the cylinder of the engine 4.
This is for the reason below. If the fuel injection device 8 is placed proximate to the cylinder head 4c of the engine 4, the temperature raised by the engine 4 heats the fuel injection device 8. This causes vapor in the fuel injected by the fuel injection device 8, causing troubles like vapor lock or breathing. The fuel injection device 8 is therefore separated as distant as possible from the cylinder head 4c of the engine 4. For example, the fuel injection device 8 is positioned at a distance of 90 mm or more from the cylinder head 4c of the engine 4.
However, placing the fuel injection device 8 apart from the engine 4 lengthens the distance of fuel transport from the fuel injection device to the combustion chamber. This deteriorates the response of the engine 4 to variation of the fuel injection by the fuel injection device 8. Furthermore, fuel injected by the fuel injection device 8 will adhere to the wall surface of the intake pipe 6 and the intake port 4a before reaching the intake valve opening 4b. The fuel adhering on the wall surface may then enter the cylinder regardless of control of the fuel injection device 8, deteriorating exhaust gas condition.
Also, with a conventional under-bone type motorcycle whose intake passage is positioned in a front part of the body, the fuel injection device is usually positioned in the rear of the intake system including the intake pipe. The fuel injection device is then not cooled by the wind that hits the motorcycle in running. The fuel injection device is least likely to be cooled especially when the motorcycle is idling or running at low speed, because the motorcycle faces a lower wind.
Therefore, a water-cooled engine is used as described in Patent Document 2. However, it is still difficult to sufficiently cool the fuel injection device.
Patent Document 3 discloses a V-type water-cooled engine equipped with an assist air control device. An assist air passage branches off from a point upstream of a throttle valve and extends to the fuel injection device, with a solenoid valve provided in the course of the assist air passage to control the amount of assist air supply. A water-temperature sensor detects the temperature of engine cooling water and the amount of assist air supply is increased when the temperature is increased.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-249028
[Patent Document 2] Japanese Patent Application Laid-Open No. 2002-37165
[Patent Document 3] Japanese Patent Application Laid-Open No. 5-33744